Concern for the environment, as well as the rising cost of energy, has resulted in a push in recent years to develop vehicles that are more fuel-efficient than previous models. Weight reduction has long been identified as an effective way to improve automotive fuel economy, such as for instance by replacing steel with lighter weight materials including aluminum and magnesium alloys. In fact, several parts of the vehicle body and powertrain are already being fabricated from aluminum alloys, including the engine block and heads, the wheels and some body panels. The alloys of aluminum that are used in body panel applications tend to be the more easily formable, but also lower strength, 5xxx series alloy. Unfortunately, structural components such as the B-pillar require the use of higher strength materials in order to satisfy roof crush and side impact safety standards.
High strength aluminum alloys are a viable alternative to the use of high strength steel in automotive structural components. In particular, heat treatable aluminum alloys such as the 2xxx, 6xxx, 7xxx and 8xxx series alloys can be hardened using an appropriate thermal treatment and artificial aging process. Due to the poor formability of such alloys in their hardened temper states, such as for instance the T6 temper state, it is common to provide a blank for forming a part in softer temper states, such as for instance T4. The blank is formed into the desired shape of the part, which is then subjected to thermal treatment and artificial aging to achieve desired mechanical properties. Unfortunately, this approach results in production times that could be impractically long, since the thermal treatment and aging processes occur over periods of time ranging from several minutes to several hours. A further problem with this approach is that warping of the shaped part may occur during the thermal treatment and aging steps, necessitating additional steps to correct the shape. As such, this approach may not be well suited to the automotive manufacturing industry, or to other high-volume applications that require the production of a large number of parts on a short time-scale.
In the article “Warm forming behavior of high strength aluminum alloy AA7075,” Trans. Nonferrous Met. Soc. China 22(2012) 1-7, Wang et al. discuss the effects of elevated temperatures on the mechanical properties of a specific 7xxx series aluminum alloy. According to Wang et al. a key element of warm forming is the preservation of the high strength temper, although the results that are presented in the article demonstrate that both the yield strength and hardness are lower after forming at a temperature between about 180° C. and 260° C. The authors of this article concluded that, in order to obtain a part without any further heat treatment, the forming temperature should not exceed 220° C. Of course, Wang et al. subjected the aluminum alloy to heat treatment lasting up to 300 s, which translates into only about twelve heating cycles per hour and is therefore impractical for use in high-volume applications.
Another study has shown that warm forming a Cu-containing AA7075 type alloy at 175° C. for one or two minutes has almost no impact on strength, and that a slight increase to 200° C. reduces the strength by about 50 MPa. This study concluded that during a common 5-step paint-bake cycle the mechanical properties of the component become uniform on a high level. Unfortunately, even with these relatively short heating times, lasting only one or two minutes, the resulting production rates are still too low to be very useful in high-volume applications such as the automotive industry.
In some applications, including the fabrication of structural components that have predetermined crush zones, it is desirable to be able to produce a part that has non-uniform mechanical properties throughout. For instance, it is advantageous to form a B-pillar with an upper end that is characterized by high mechanical strength and a lower end that is characterized by relatively lower mechanical strength. During a collision, some of the force of the impact is absorbed when the lower end of the B-pillar deforms, resulting in improved protection for the occupants of the vehicle. Unfortunately, the processes that are currently being used to form shaped parts from aluminum alloy blanks in hardened temper states do not support the tailoring of mechanical properties in this way.
It would therefore be desirable to provide a process that overcomes at least some of the above-mentioned limitations and disadvantages of the prior art.